bump ubuntu back up to 20 for Python 3.10, remove clang 6 and gcc 5/6 and add gcc9 to test matrix

Author: sbSteveK

.github/workflows/ci.yml

@@ -11,7 +11,7 @@ env:
   BUILDER_SOURCE: releases
   BUILDER_HOST: https://d19elf31gohf1l.cloudfront.net
   PACKAGE_NAME: aws-crt-python
-  LINUX_BASE_IMAGE: ubuntu-18-x64
+  LINUX_BASE_IMAGE: ubuntu-20-x64
   RUN: ${{ github.run_id }}-${{ github.run_number }}
   CRT_CI_ROLE: ${{ secrets.CRT_CI_ROLE_ARN }}
   AWS_DEFAULT_REGION: us-east-1
@@ -196,15 +196,13 @@ jobs:
       fail-fast: false
       matrix:
         compiler:
-          - clang-6
           - clang-8
           - clang-9
           - clang-10
           - clang-11
-          - gcc-5
-          - gcc-6
           - gcc-7
           - gcc-8
+          - gcc-9
     permissions:
       id-token: write # This is required for requesting the JWT
     steps: